Method for producing isoprene

ABSTRACT

ISOPRENE IS PRODUCED BY CONTACTING AT AN ELEVATED TEMPERATURE A GASEOUS MIXTURE CONTAINING TERT-BUTYL METHYL ETHER AND OXYGEN WITH A SUITABLE CATALYST CONTAINING, AS AT LEAST ONE COMPONENT, A COMPOUND COMPRISING OXYGEN AND ONE OR MORE MEMBERS SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, VANADIUM, TUNGSTEN AND URANIUM.

United States Patent US. Cl. 260681 Claims ABSTRACT OF THE DISCLOSUREIsoprene is produced by contacting at an elevated temperature a gaseousmixture containing tert-butyl methyl ether and oxygen with a suitablecatalyst containing, as at least one component, a compound comprisingoxygen and one or more members selected from the group consisting ofmolybdenum, vanadium, tungsten and uranium.

This invention relates to a novel process for producing isoprene bycontacting with a suitable catalyst a gaseous mixture containing oxygenand tert-butyl methyl ether [(CH COCH hereinafter abbreviated as TBME].

Isoprene is a useful compound as a starting material for syntheticrubbers, polyisoprene rubbers and copolymer rubbers which are underdevelopment in recent years, and the establishment of an economicalproduction process thereof has been desired.

The present inventors have found an advantageous isoprene synthesisprocess, in which a gaseous mixture con taining TBME and oxygen iscontacted with a solid catalyst to produce isoprene directly.

More particularly, the invention is concerned with a process forproducing isoprene directly by contacting a gaseous mixture containingTBME and oxygen in a suitable ratio with a solid catalyst containing, asat least one component, a compound comprising oxygen and one or two ormore members selected from the group consisting of molybdenum, tungsten,vanadium and uranium.

TBME, which is a starting material employed in the present invention,can be easily prepared by reacting isobutylene with methanol in aliquid-orgas phase in the presence of a suitable catalyst. Forfexample,a mixture comprising isobutylene, methanol and a small amount ofsulfuric or phosphoric acid is heated at about 100 C. under a pressurenecessary for maintaining the mixture in a liquid phase, whereby TBMEcan be obtained in a high yield. On the other hand, US. Pat. 3,135,807(1964), for example, teaches that a gaseous mixture containingsubstantially equimolar amounts of isobutylene and methanol is passed,at a temperature of 110 C. under a pressure of 15 kg./cm. through asolid catalyst having a composition of MoO -Bi O SiO whereby TBME can beobtained substantially quantitatively with a conversion of 50%.

It is not always necessary to use pure isobutylene for the synthesis ofTBME, because methanol reacts selectively with only isobutylene in the C-mixture obtained by thermal cracking of naphtha. Boiling point of theTBME is higher than that of unreacted substance and hence TBME can beeasily separated therefrom by simple distillation. g

Methanol and TBME form an azeotropic mixture and it is difficult toseparate completely TBME from methanol by simple distillation. However,production of isoprene of the present invention is not disturbed by thepresence of methanol and it is not necessary to use TBME completely freefrom methanol. Thus, in the present invention, the starting materialTBME can not only be easily obtained Patented Apr. 13, 1971 from anindustrially inexpensive C -mixture and methanol, but also can be usedwithout any particular purification. These points are extremelyadvantageous from the point of view of the costs of starting materials.

As the oxygen source, any oxygen-containing gas may be used. Ordinarily,however, air is used as an inexpensive oxygen source. The mixing ratioof oxygen to TBME is not irrelevant to the yield of isoprene but can beselected considerably freely.

If necessary, sometimes other inert gases, for example, steam, carbondioxide etc. can be used like as nitrogen in order to control thereaction conditions.

Catalysts usable in the present invention are those which contain, as atleast one component, a compound comprising oxygen and one or moremembers selected from the group consisting of tungsten, vanadium,molybdenum and uranium (such catalysts will be referred to as catalystsA, hereinafter). Examples of the catalyst (A) include tungsten (IV, VI)oxide, vanadium (II, III, IV, V) oxide, molybdenum (IV, VI) oxide,uranium (IV, VI) oxide and the like.

Star-ting materials for preparing catalyst (A) are those shown above orsubstances capable of being converted into the catalysts (A) by thermaldecomposition, such as for example, ammonium heptamolybdate, molybdicacid, ammonium metavanadate, ammonium tungstate, uranyl nitrate anduranyl acetate.

In addition to said main components, promoters may be added in order toinhibit side reactions and to increase selectivity. For example,compounds of phosphorus, sulfur, boron, antimony, bismuth, tellurium,silver, barium, calcium, magnesium, potassium and sodium are effectivefor said purpose. (Catalysts incorporated with these promotors will bereferred to as catalysts B, hereinafter). The proportions of theabove-mentioned elements in the catalysts can be optionally selected,though said proportions are not irrelevant to conversion andselectivity.

These catalysts A and B may be used after shaping by compression,calcination and the like procedures, without using any carriers or in astate supported on carriers. As the carriers, molten or semi-moltentitanium oxide, zinc oxide, tellurium oxide, alumina and active carbonpumice are ordinarily used. Especially a compound of silicon and oxygen(C) (such as silica gel) is preferable as the carrier. Among theabove-mentioned catalysts (A), there are some which display markedisoprene-forming ability only when used in combination with the compound(C). Such synergistic effect is observed not only in the case where thecatalysts (A, B) are supported on the compound of silicon and oxygen,but also in the case Where the catalysts (A, B) and the compound (C) areindividually finelydivided and finely mixed together, and the mixture iscompressed for use as a catalyst; and also in the case where a catalystis prepared by gelling the catalysts (A, B) together with a silica solin the form of a colloidal dispersion, followed by calcination.

There is a maximum value in isoprene yield depending on the mixing ratioof (A, B) to (C) in the catalyst employed, and the properties and kindsof the starting materials (A, B) and (C), and can be selectedfrom therange of 1:99 to 99:1. Generally, a range of from 5:95 to 9525 givesfavorable results.

In order to further enhance the above-mentioned effect, the presentinventors studied the influence of the com pound (C). As the result, theinventors have found that When silica gel is to be used as a startingmaterial, the use of silica gel having a surface area of less than 350m. g. can greatly increase the isoprene selectivity.

It is well known, in general, that even in the case of substancescomprising silicon and oxygen, the physical and chemical propertiesthereof are markedly complex, and

that they delicately vary in surface area, average pore diameter, watercontent, intensities and kinds of acid sites, depending on differencesin preparation conditions.

Especially the surface area of silica gel employed as a startingmaterial has an important influence on isoprene selectivity, and use ofa silica gel small in surface area is preferred.

In the next place, the pro'motors employed in the present process willbe explained.

In case the promotors are phosphorus compounds, the catalysts may beprepared, for example, either by mixing phosphomolybdic acid, vanadiumphosphate or phosphotungstic acid with silica gel, or by mixingmolybdenum trioxide and a suitable amount of phosphoric acid with silicagel. By addition of phosphorus or a compound of phosphorus and oxygen,the isoprene selectivity based on isobutylene and the amount of isopreneproduced per unit volume, time of catalyst are increased, and thedeposition of carbon on the catalyst surface is inhibited.

Sulfur or sulfur compounds also display substantially the same elfect asthat of phosphorus. In addition thereto, boron or a compound comprisingboron and phosphorus, e.g. boron phosphate, shows substantially the sameeffect. In addition to such substances as phosphorus, sulfur and boronwhich give acidic properties, there may be used, as promotors, suchsubstances which conversely give basic properties, e.g. compounds ofalkali or alkaline earth metals such as sodium, potassium, calcium,barium and magnesium. These substances, when added in small amounts,have actions to increase the isoprene selectivity. Further, tellurium,antimony or bismuth, when incorporated in a small amount, displays anaction to make catalyst activity extremely mild, and hence can be usedas an activity-controlling agent.

The contact time employed in the present process can be selected fromsuch an extremely wide range as from 0.1 to 100 seconds. The optimumcontact time varies depending on the kind of catalyst and thecomposition of starting gas, but a contact time Within the range of 0.5to 30 seconds gives favorable results, in general.

The reaction temperature also varies depending upon the kind of thecatalyst employed and the composition of the starting gas. If thetemperature is excessively low, the reaction rate becomes insuificient,while if the temperature is excessively high, the isoprene selectivityis lowered. Generally the reaction temperature is 100 500 0., preferably150350 C.

The reaction according to the present process may be effected under anyof reduced or pressurized conditions. However, the reaction is carriedout rather preferably under pressurized conditions.

The catalyst bed employed in the present process may be any type of afixed bed or of a fluidized bed.

The reaction product comprises, in addition to isoprene, isobutylene andmethyl alcohol which are decomposition products of TBME, and a smallamount of formaldehyde and unreacted TBME. The catalyst of the presentinvention can be used in the production of isoprene by reaction offormaldehyde and isobutylene or of methanol, oxygen and isobutylene,therefore after separation of the isoprene, the isobutylene, methylalcohol and formaldehyde, which are in a mixed state, can be recycled tothe reactor together with freshly fed TBME and oxygen (or air).

In the isoprene synthesis reaction according to the present invention,the ratio of oxygen to TBME is very important, as seen also in Example 7set forth later. If the amount of oxygen is too much, a completecombustion of TBME is greatly progressed and the yield of isoprene ismarkedly lowered, as this is naturally anticipatable. If the amount ofoxygen is too small, isobutylene and methanol are formed mainly and theyield of isoprene is lowered. Employment of decreased amount of oxygenresults in formation of a mixture of isobutylene and methanol whichcontains small amounts of isoprene and formaldehyde. This phenomenonsuggests a novel and advantageous process for inexpensively separatingisobutylene through TBME from a mixed C -fraction. However, so far asthe process is deemed as an isoprene synthesis process, the O /TBMEratio is preferably about 0.2-1.5 (molar ratio).

The present invention will be illustrated below with reference toexamples. Definitions of the yields and recovery ratios shown in theexamples are as follows:

Isoprene yield (percent) (Mole of formed isoprene) X 100 (Mole of fedTBME) Formaldehyde yield (percent) (Mole of formed formaldehyde) (Moleof fed TBME) X100 TBME conversion (percent) (Mole of fed TBME) (Mole ofrecovered TBME) X 100 (Mole of fed TBME) Methyl alcohol recovery ratio(percent) (Mole of recovered methyl alcohol) X 100 (Mole of fed TBME)EXAMPLES 1-4 Molybdenum trioxide, vanadium pentoxide, tungsten trioxide,uranium trioxide and a silica catalyst (N 608, a product of Nikki KagakuK.K.) were finely divided to less than 100 mesh. The individual metaloxides and the silica catalyst were thoroughly mixed in such ratios asset forth in Table 1, in the presence of a small amount of water. Eachof the thus obtained mixtures was compressionrnolded into tablets, wasfired at 400 C. for 8 hours, and was then ground to 12-16 mesh toprepare a catalyst.

15 ml. of the thus prepared catalyst was charged into a quartz reactiontube of 15 mm. in inner diameter, and oxidation was effected, whileintroducing a starting gas comprising 28% by volume of TBME and 72% byvolume of air, at such a temperature as shown in Table 1 andsubstantially at atmospheric pressure for a contact time of 10 seconds.

The resulting gas was passed to a trap maintained at C., and a liquidcondensate and a non-condensate were subjected to measurement in amountand to analysis according to gas-chromatography, respectively, tocalculate the amounts of reaction products and unreacted sub 65 stances.The results were as shown in Table 1.

TABLE 1 Percent Methyl Reaction Isobutylene alcohol temperature IsopreneTBME recovery recovery Example Catalyst composition C.) yield conversionratio rat MOO ZSlOz(1:2) 250 10. 3 97. 8 77 60. 6 :SiO (1- 230 8. 5 98.5 7B 51. 5 W03: SiOz (1 1). 250 7. 0 97. 0 77 62. 2 VOzzSlO; (1: 270 8 597.0 80 57. 1

EXAMPLE 5 1.5 g. of phosphoric acid, 23 g. of molybdenum trioxide, 6 g.of concentrated nitric acid and 58 g. of bismuth nitrate [Bi(NO -5H O]were dissolved in 60 g. of water. This solution was added to 80 g. of asilica sol (Snowtex produced by Nissan Kagaku K.K.; SiO content 32%),and the mixture was stirred by means of a mixer and was then vaporizedto dryness over a water bath. Subsequently, the mixture was fired at 600C. for 5 hours and was then ground to 12-16 mesh to prepare a catalyst.

15 ml. of the thus prepared catalyst was charged into a quartz reactiontube, and oxidation was eifected, while introducing a starting gascomprising 20% by volume of TBME and 80% by volume of air, at atemperature of 260 C. and substantially at atmospheric pressure for acontact time of seconds. The product was quantitatively analyzed in thesame manner as in Example 1 to obtain the following results:

Percent Isoprene yield 16.8 TBME conversion 97.6 Isobutylene recoveryratio 69.1 Methyl alcohol recovery ratio 58.5

EXAMPLE 6 121 g. of bismuth nitrate [Bi(NO -5H O] was dissolved in asolution comprising 600 cc. of water and 50 cc. of concentrated nitricacid to form an aqueous bismuth nitrate solution. Separately, 62.5 g. oftungstic acid and 14.7 g. of ammonium molybdate were dissolved in asolution comprising 100 cc. of 28% ammonia water and 500 cc. of water toform a solution. The two solutions thus formed were quickly mixed witheach other to deposit a precipitate, and the mixed solution was adjustedto pH 5 by use of nitric acid and ammonia water. The precipitate waswater-washed and filtered, and the resulting cake Was thoroughly kneadedwith 270 g. of a silica sol (Snowtex N produced by Nissan Kagaku K.K.;Si0 content 30%). Subsequently, the mixture was vaporized to drynessover a water bath at 100 C. and was then fired at 500 C. for 6 hours toprepare a catalyst containing Mo, W, Bi and Si.

The thus prepared catalyst was ground to 12-16 mesh, and 20 cc. of theground catalyst was charged into an SUS-made, U-shaped reaction tube of18 mm. in inner diameter and was heated over a salt bath at 280 C. Tothe above-mentioned catalyst layer was passed at atmospheric pressurefor a contact time of about 4 seconds a starting gas comprising 20% ofTBME, 20% of isobutylene and 60% of air. The product was quantitativelyanalyzed in the same manner as in Example 1 to obtain the followingresults:

Percent Isoprene yield 26.0 TBME conversion 99.5 Methyl alcohol recoveryratio 49.8

EXAMPLE 7 Catalyst employed: MoO -0.5V O 6WO '3Bi O /SiO Salt bathtemperature: 270 C.

SV: 1000/hr.

Under the above conditions and While varying the amount of oxygen, thesame operations as in Example 1 were effected to obtain the resultsshown in the following table:

=*No1'n.'lhe mark shows that the salt bath temperature was C.

In each of the above reactions, N was made present in an amount of about4 times the mole of 0 What we claim is:

. 1. A process for synthesizing isoprene which comprises contacting atan elevated temperature a gaseous mixture containing tert-butyl methylether and oxygen with a solid catalyst containing, as at least onecomponent, a compound comprising oxygen and one or more members selectedfrom the group consisting of molybdenum, vanadium, tungsten and uranium,wherein the molar ratio of oxygen to tert-butyl methyl ether is withinthe range of from 0.2 to 1.5.

2. A process according to claim 1, wherein the mixed gas containingtert-butyl methyl ether and oxygen is incorporated with isobutylene andmethanol which have been formed as by-products in the precedingreaction.

3. A process according to claim 1, wherein as a carrier for the catalystis used a substance composed mainly of silicon and oxygen.

4. A process according to claim 1, wherein the reaction is effected at atemperature within the range of from 150 C. to 350 C.

5. A process according to claim 1, wherein the tertbutyl methyl ether isobtained in such a manner that isobutylene contained in a mixed C-fraction is allowed to react with methanol to form tert-butyl methylether, which is then separated from other C -fractions.

References Cited UNITED STATES PATENTS 8/1964 Habeshaw et a1. 260681 5/1966 Yanagita et a1. 260--681 US. Cl. X.R.

